Planetary Processes

The global movement of a planet's atmosphere, driven by uneven heating and the planet's rotation — the banded jets and great storms of the gas giants, the super-rotating winds of Venus that lap the planet in days, and the planet-wide dust storms of Mars.

The gradual loss of a planet's atmosphere to space — light gases escaping thermally, the solar wind stripping the air from an unmagnetised planet, or impacts blasting it away. It is why Mars, which lost its magnetic field, also lost most of its once-thick atmosphere.

How a planet's climate changes over billions of years — the runaway greenhouse that turned Venus into a furnace, the slow drying and cooling of Mars, and the feedbacks that have kept Earth habitable. Comparing the three is the key to understanding what makes a climate stable.

'Ice volcanism' — the eruption not of molten rock but of water, ammonia, or methane from the interior of a cold, icy world. Enceladus jets plumes of water ice into space, and Triton has erupting geysers; the erupted material is these worlds' equivalent of lava.

The record of impacts written across a planet's surface, and the way it is read: because craters accumulate over time, a more heavily cratered surface is older. Counting craters is the main way the ages of surfaces across the Solar System are estimated.

The protection a planet's magnetic field gives its atmosphere by deflecting the solar wind around it. Earth's magnetosphere shields its air; Mars, having lost its global field billions of years ago, was left exposed to the stripping that thinned its atmosphere.

The separation of a young, molten planet into layers by density — the heaviest metal sinking to form a core, the lightest material rising to form a crust. It sets the deep structure every other process then acts on.

The slow motion of a planet's rigid surface plates over its convecting mantle, continually creating and recycling crust. Earth is the only body where plate tectonics is known to operate today, and it is deeply tied to Earth's long-term climate and habitability.

The warming of a planet's surface by an atmosphere that lets sunlight in but traps the outgoing heat. Modest and life-enabling on Earth, it is extreme on Venus, whose thick carbon-dioxide atmosphere makes it the hottest planet in the Solar System.

The eruption of molten rock from a planet's interior onto its surface — from Earth's volcanoes to the relentless silicate volcanism of Jupiter's moon Io, the most volcanically active body in the Solar System, whose surface and plumes are painted with sulphur and sulphur-dioxide frosts.